1. Field of the Invention
The present invention relates generally to transmission of broadcast television signals using non-linear devices as high power amplifiers, and more particularly to methods and systems for precorrecting input signals before they are inputted to a non-linear amplifying device for transmission to an antenna for propagation.
2. Background and Related Art
It is generally known in the art to compensate for distortions introduced to input signals (such as composite television signals) by non-linear devices used to amplify such signals, by precorrecting such signals prior to inputting them to the non-linear device. See, e.g., U.S. Pat. No. 5,198,904.
The benefits of inductive output tubes (IOTs) as power amplifiers in television transmitters, especially in common amplification configurations, are well known in terms of increased efficiency and reliability. However, a characteristic of the IOT which must be compensated for is its nonlinearity.
A correction scheme known as Digital Adaptive Correction (DAP) has been proposed to compensate for signal distortions caused by such non-linear amplifying devices. According to DAP, the output signal of a non-linear amplifier is measured and compared with an ideal signal at the output of a modulator. The difference between the ideal signal and the measured output signal represents error caused by the non-linear amplifier. This difference could be stored, such as in a Look-Up Table (LUT), against magnitude and/or phase values of the input signal. Thereafter, signals being inputted to the modulator would be measured, the appropriate correction value would be retrieved from the LUT, and applied to the input signal before being amplified by the non-linear device.
A significant problem exists with the proposed DAP correction scheme in that it assumes that the non-linear characteristic of the amplifying device remains the same over the entire frequency band of the input signal to be amplified. The present inventor has observed, however, that in some IOT devices, the non-linearity actually varies over the frequency band of interest. The proposed DAP correction scheme is unable to compensate for such frequency-varying non-linearity, because it is a one-dimensional correction, taking into account only the amplitude of the input signal at any sampling point in determining the appropriate correction value to add to the input signal before it is amplified.
There thus exists a need in the art to provide a correction method and system that takes into account the frequency-varying nature of the characteristics of non-linear amplifying devices such as IOTs, as used particularly in television transmitters.